A polypropylene-based resin in-mold expansion molded article is superior in chemical resistance, heat resistance, shock absorption and compressive strain rebound as compared to a polystyrene-based resin in-mold expansion molded article, and is also superior in heat resistance and compressive strength as compared to a polyethylene-based resin in-mold expansion molded article. Therefore, such polypropylene-based in-mold expansion molded articles are used widely as shock absorbing packaging materials, returnable containers and automotive parts.
Particularly, since polypropylene-based resin in-mold expansion molded articles can be molded flexibly as various shapes of shock absorbing packaging materials in conformity with the shapes of commercial goods and parts to be packaged without requiring cutting work, the polypropylene-based resin in-mold expansion molded articles are used widely for electronic machines, industrial materials and the like.
Polypropylene-based resin molded articles can be molded in various shapes. However, because the temperature range for molding to obtain satisfactory products is narrow as compared to a case of polystyrene or the like, engineers must be skilled in molding techniques including regulation of the heating steam pressure, regulation of the heating time and regulation of the cooling time in molding. When a molded article of complicated shape should be obtained, if there is such a thin and narrow shaped portion that is a so-called thin-wall portion to accommodate only a few pre-expanded particles along the thickness direction, it is difficult to obtain a satisfactory shape and satisfactory surface appearance. Consequently, in such a portion, failures such as insufficient shock absorption and insufficient strength and the poor fusion of the pre-expanded particles may occur easily, thus imposing severe constraints on the shape design. As for the in-mold expansion molding using a pre-expanded particulate polypropylene-based resin, generally, due to the use of raw materials having low resin melting points, the secondary expandability (secondary expansion ratio) tends to increase when steam heating is carried out. As a result, for shaping a thin-wall, the use of a resin having a low melting point can be a measure for solving the above-described problem. However, the surfaces of the molded articles tend to have plenty of wrinkles and the rebound of the molded articles from the shrinkage after molding is insufficient in many cases. Also in a molding process for obtaining box-shaped molded articles, a so-called “fall-inward” phenomenon occurs easily. The “fall-inward” means a phenomenon in which a difference is raised between the edge portion dimension and the central portion dimension in a box-shaped molded article. The absolute value of this difference varies depending on the sizes of individual designed products. When the fall-inward is large, the molded articles are defective and not useful products.
In view of the above-described problems, for example, a method of modifying a resin surface has been proposed (Patent Document 1). According to this method, for obtaining pre-expanded particles having preferable secondary expandability and fusion property from pre-expanded polyolefin-based particles for in-mold expansion molding, a polypropylene-based resin is dispersed in a dispersion medium containing an organic peroxide. However, in this method, equipment to deal with the metal corrosion by the organic peroxide is required, and the dispersion medium tends to be non-uniform, and the product quality tends to be varied.
For the purpose of improving secondary workability, there is disclosed a method of using a mixture of a polypropylene-based resin and a propylene-α-olefin-based resin having a specific Vicat softening point (Patent Document 2). However, this method shows no effects of improving the secondary expandability and fusion property and suppressing the fall-inward.
In Patent Document 3, it has been found that a pre-expanded particulate polypropylene-based resin based on a resin having a specific melt index obtained by mixing resins each having a specific melt index is satisfactory with respect to surface appearance and fusion, but no specific description is presented as to whether such a pre-expanded particle can be applied or not to a molded article having a thin-wall shape requiring a higher secondary expandability and a higher fusion property.    [Patent Document 1] JP 2002-167460 A    [Patent Document 2] JP H10-251437 A    [Patent Document 3] JP 2000-327825 A